Area codes 815 and 779
telephone area codes
Area codes 815 and 779 is a telephone area code in the North American Numbering Plan (NANP) serving the state of Illinois in the United States. It covers the northwestern part of Illinois, including cities such as Rockford, Joliet, and DeKalb. 815 was created in 1947 as one of the original area codes assigned to Illinois. It was overlaid by 779 in 2007.